Combining light sources of different spectra permit lighting devices to emit a light spectrum of almost any desired energy content. For example, red light can be combined with unsaturated green light to yield a light spectrum that renders colors similar to daylight or similar to incandescence depending on the amount of accompanying blue light. Using red, green, and blue light sources, colors from such sources can be combined in any proportion to yield any aggregate color within the gamut of colors.
Color is the visual effect that is caused by the spectral composition of the light emitted, transmitted, or reflected by objects. Human vision is primarily related to color and brightness (contrast) of the light source, and (if reflected light is present) the spectrum that is reflected from an object being illuminated.
As a heated object becomes incandescent, it first glows reddish, then yellowish, then white, and finally bluish. Thus, apparent colors of incandescing materials are directly related to their actual temperatures (in Kelvin (K)). Practical materials that incandesce are said to have correlated color temperature (CCT) values that are directly related to color temperatures of blackbody sources. CCT is measured in Kelvin (K) and has been defined (e.g., by the Illuminating Engineering Society of North America (IESNA)) as “the absolute temperature of a blackbody whose chromaticity most nearly resembles that of the light source.” Light having a CCT below 3200K is yellowish white in character and is generally considered to be warm white light, whereas light having a CCT between 3200K and 4000K is generally considered to be neutral white light, and light having a CCT above 4000K is bluish white in character and generally considered to be cool white light.
Aspects relating to the present disclosure may be better understood with reference to the 1931 CIE (Commission International de l'Eclairage) Chromaticity Diagram, which maps out human color perception in terms of two CIE parameters x and y. The 1931 CIE Chromaticity Diagram is reproduced at FIG. 1. The spectral colors are distributed around the edge of the outlined space, which includes all of the hues perceived by the human eye. The boundary line represents maximum saturation for the spectral colors. The chromaticity coordinates (i.e., color points) that lie along the blackbody locus (“BBL”) obey Planck's equation: E(λ)=A λ−5/(eB/T−1) where E is the emission intensity, λ is the emission wavelength, T the color temperature of the blackbody, and A and B are constants.
Quality artificial lighting generally attempts to emulate the characteristics of natural light. Natural light sources include daylight with a relatively high CCT (e.g., ˜5000K) and incandescent lamps with a lower CCT (e.g., ˜2800K).
Solid state light emitters such as LEDs typically emit narrow wavelength bands. Such emitters include or can be used in combination with lumiphoric materials (also known as lumiphors, with examples including phosphors, scintillators, and lumiphoric inks) that absorb a portion of emissions having a first peak wavelength emitted by the emitter and re-emit light having a second peak wavelength that differs from the first peak wavelength.
Light perceived as white or near-white may be generated by a combination of red, green, and blue (“RGB”) emitters, or, alternatively, by combined emissions of a blue LED and a lumiphor such as a yellow phosphor (e.g., YAG:Ce or Ce:YAG). In the latter case, a portion of the blue LED emissions passes through the phosphor, while another portion of the blue emissions is downconverted to yellow, and the blue and yellow light in combination are perceived as white.
Depending on the combination of LEDs and/or lumiphors used, aggregate emissions of a solid state device may be under-saturated with certain colors of the spectrum or oversaturated with certain colors.
Color reproduction is commonly measured using Color Rendering Index (CRI) or average Color Rendering Index (CRI Ra). To calculate CRI, the color appearance of 14 reflective samples is simulated when illuminated by a reference radiator (illuminant) and the test source. The general or average color rendering index CRI Ra is a modified average utilizing the first eight indices, all of which are pastel colored with low to moderate chromatic saturation. (R9 is one of six saturated test colors not used in calculating CRI, with R9 embodying a large red content.) CRI and CRI Ra are used to determine how closely an artificial light source matches the color rendering of a natural light source at the same CCT. Daylight has a high CRI Ra (approximately 100), with incandescent bulbs also being relatively close (CRI Ra greater than 95), and fluorescent lighting being less accurate (with typical CRI Ra values of approximately 70-80).
CRI Ra (or CRI) alone is not a satisfactory measure of the benefit of a light source, since it confers little ability to predict color discrimination (i.e., to perceive subtle difference in hue) or color preference. There appears to be a natural human attraction to brighter color. Daylight provides a spectrum of light that allows the human eye to perceive bright and vivid colors, which allows objects to be distinguished even with subtle color shade differences. Accordingly, it is generally recognized that daylight and blackbody sources are superior to many artificial light sources for emphasizing and distinguishing color. The ability of human vision to differentiate color is different under CCT conditions providing the same CRI Ra. Such differentiation is proportional to the gamut of the illuminating light.
Gamut area of a light source can be calculated as the area enclosed within a polygon defined by the chromaticities in CIE 1976 u′v′ color space of the eight color chips used to calculate CRI Ra when illuminated by a test light source. Gamut area index (GAI) is a convenient way of characterizing in chromaticity space how saturated the illumination makes objects appear—with a larger GAI making object colors appear more saturated. GAI is a relative number whereby an imaginary equal-energy spectrum (wherein radiant power is equal at all wavelengths) is scored as 100. GAI for a test source is determined by comparing color space area of the light being tested to the color space area produced by the imaginary or theoretical equal-energy spectrum (EES) source. Unlike CRI Ra (or CRI), which has a maximum value of 100, GAI can exceed 100, meaning that some sources saturate colors more than an equal-energy source serves to saturate color.
It is found that typical blackbody-like light sources and typical daylight-like light sources have different gamut areas. Low CCT sources (e.g., incandescent emitters) have a GAI of approximately 50% (i.e., about half the gamut area of the EES source). Sources with higher CCT values have a larger GAI. For example, a very bluish light with a CCT of 10000K may have a GAI of 140%.
Another way of characterizing how saturated an illuminant makes objects appear is relative gamut area, or “Qg” (also referred to as “Color Quality Scale Qg” or “CQS Qg”), which is the area formed by (a*, b*) coordinates of the 15 test-color samples in CIELAB normalized by the gamut area of a reference illuminant at the same CCT and multiplied by 100. In a manner similar to GAI, Qg values can exceed 100; however, Qg values are scaled for consistency relative to CCT. Because of chromatic adaptation, and because CCT is selected to set the overall color tone of an environment as part of the lighting design process, variable-reference measures such as Qg may be especially relevant to applied lighting design. If the relative gamut is greater than that of the reference, and if illuminance is lower than that provided by daylight, then an increase in preference and discrimination might be expected relative to the reference at that same CCT. Conversely, if the relative gamut is smaller than that of the reference, then a decrease in preference and discrimination might be expected relative to the reference at the same CCT.
It is believed that, in at least certain contexts, some consumers may prefer light sources with significantly enhanced vividness. It may be challenging to provide enhanced vividness in combination with high luminous efficacy, and further in combination with reasonably high color rendering index values.
It is important that lighting be of appropriate intensity for the task at hand and also have appropriate color rendering characteristics. For most daytime tasks, light sources (whether artificial or natural) should have high intensity and high color rendering. Conversely, for sleeping, light should have very low levels. The color differentiation of night vision is very low.
Light affects human circadian rhythms. Human physiology responds non-visually to the presence or absence of certain wavelengths. For example, blue light is known to suppress melatonin, and ultraviolet rays are known to damage the skin. The intensity of light and the spectral content of light have a strong effect on the human circadian rhythms. These circadian rhythms are ideally synchronized with the natural light.
Circadian rhythm disorders may be associated with change in nocturnal activity (e.g., nighttime shift workers), change in longitude (e.g., jet lag), and/or seasonal change in light duration (e.g., seasonal affective disorder, with symptoms including depression). In 2007, the World Health Organization named late night shift work as a probable cancer-causing agent. Melatonin is an anti-oxidant and suppressant of tumor development; accordingly, interference with melatonin levels may increase the likelihood of developing cancer. Methods involving stimuli with artificial light sources to modify the phase and amplitude of a human circadian cycle (e.g., for cycle resetting) have been developed, such as disclosed in U.S. Patent Application Publication No. 2006/0106437A1 to Czeisler et al.
Artificial light sometimes includes too much blue light in the evening, which suppresses melatonin and hinders restful sleep. Exposure to artificial light during the night may inhibit a person from falling to sleep or returning to sleep, and may also cause a temporary loss of night vision. It is principally blue light (e.g., including blue light at a peak wavelength value between 460 to 480 nm, with some activity from about 360 nm to about 600 nm), that suppresses melatonin and synchronizes the circadian clock, proportional to the light intensity and length of exposure. As shown in FIG. 2, the action spectrum for melatonin suppression (with six individual data points represented as black squares) shows short-wavelength sensitivity that is very different from the known spectral sensitivity of the scotopic response curve (represented with a solid line) and photopic response curve (represented with a dashed line).
Natural light varies with respect to intensity and/or CCT depending on season, latitude, altitude, time of day, and weather conditions. Natural light also varies each day with respect to intensity and CCT. The changing CCT of sunlight over the course of a day is mainly a result of scattering of light, rather than changes in black-body radiation. Ignoring variations due to weather conditions, natural light intensity typically is low at sunrise, increases through mid-morning to a high level at mid-day, and then decreases in mid-afternoon to evening to a low level at sunset. CCT also varies in a predicable manner. During sunrise and sunset, CCT tends to be around 2,000K; an intermediate CCT value of around 3,500K is exhibited shortly after sunrise or before sunset (when daylight is redder and softer compared to when the Sun is higher in the sky); and a CCT of around 5,400K is exhibited around noontime. Color temperatures for various daylight sources are tabulated in FIG. 3. Low (or warm) CCT values are consistent with reduced blue content, while higher (or cool) CCT values are consistent with increased blue content.
Generally, a light that is dim and exhibits a low (warm) CCT promotes restfulness (e.g., such as may be desirable in the evening and night before sleep), and a light that is bright and exhibits a high (cool) CCT promotes alertness (such as may be desirable in the morning and during the day). A light having a very low intensity and a very low CCT would least interfere with a person returning to sleep after being awakened in the middle of the night.
Color changing lights are known in the art. One example of a color changing light bulb is the Philips “Hue” bulb (Koninklijke Philips N.V., Eindhoven, the Netherlands), which is understood to include an array of red LEDs, blue LEDs, and blue shifted green LEDs (each including a blue LED arranged to stimulate emissions of a green phosphor to provide very saturated green color). Such bulbs permit different colors, CCTs, and/or intensities of light to be selected by a user via a computer or portable electronic device.
Despite the availability of color changing lamps, such lamps have limitations that inhibit their utility. It can be difficult for users to program and/or operate lighting devices to obtain desired illumination conditions that take into account temporal variations in natural light. Avoiding potential interference with circadian rhythms without unduly sacrificing perceived light quality is another concern. It can also be difficult to provide vivid illumination in combination with high color rendering at a desired color point. Still another concern includes maintaining high luminous efficacy over a variety of illumination conditions. Additional concerns include ease of control by one or more users. It can also be difficult for users to program lighting devices to obtain desired illumination conditions that take into account variations in natural light that may be attributable to multiple factors such as the season, latitude, time of day, and weather conditions.
The art continues to seek improved lighting devices and methods that address limitations of conventional lighting devices and methods.